This invention relates to an electrostatic attracting sheet which is used in electrostatic holding apparatuses adapted to hold and transport paper sheets in the so-called X-Y plotters, copying machines or the like.
The known electrostatic holding apparatuses utilize the electrostatic coulomb force in order to attract a paper sheet or other object to be held on the holding apparatuses. Said electrostatic holding apparatuses each comprises an insulating base layer, a pair of electrodes formed thereon and an electrostatic attracting sheet laminated on and covering the base layer and electrodes. The attracting sheet is of a predetermined conductivity or of a limited electrical resistance. In said apparatuses, positive and negative electrostatic charges are imparted to the electrodes, respectively, to induce electrostatic charges in the attracting sheet.
Some examples of the known attracting devices are disclosed in the Japanese Patent Publications TOKKOSHO-55-20830 and -57-58872.
The electrostatic attracting sheets in said known devices have a structure such that carbon black or other powder of electrically conducting materials is dispersed and embedded in a sheet of plastics or synthetic rubber. Rolling technique or extrusion method is used to form the sheets.
However, it has been difficult to disperse the conducting powder homogeneously in the plastics or rubber. The attracting sheets made by those method are disadvantageous because their conductivity is not well controlled but is likely to be influenced by ambient temperature or humidity. Thus, they cannot exert their attracting force in a stable manner.
Therefore, the inventions disclosed in the Japanese Patent Publications are directed to an improvement to diminish such disadvantages in the known art, the improvement residing in the adding of a stabilizing agent, a plasticizer, anti-aging agent, a lubricant and/or other special additives. Such additives added to the electrically conducting material are intended to eliminate the bad influences of ambient temperature or humidity in order to stabilize the attracting force.
In spite of such an intention, a well stabilized attracting force of a desirable level has not been achieved up to now. Besides, such special additives have caused a considerable rise in manufacturing cost.